The present invention relates to a magnetic recording/reproducing apparatus of the helical scanning system using rotating heads, and more particularly to a magnetic recording/reproducing apparatus having a head structure suitable for selectively recording and reproducing either of two kinds of signals containing different amounts of information in the ratio of two to one.
An example of a signal including two kinds of signals with diffrent amounts of information is the digital picture signal. There are two kinds of digital picture signal. One is a digital picture signal of the 525-line/60-field system or the 625-line/50-field system (hereafter referred to as the current digital picture signal), and the other is a high-quality digital picture signal of the 1125-line/60-field system, for example, (hereafter referred to as the HD digital picture signal).
At present, as recording apparatus of the current digital picture signal, the so-called D1-VTR, D2-VTR, etc. have been put into practical business use. With regard to the D1 format for the D1-VTR and the D2 format for the D2-VTR, description has been made in "Television Gakkai-Shi" (the Journal of the Institute of Television Engineers of Japan ), Vol. 42, No. 4 (April, 1988), pp. 338-346. As regards the recording apparatus of the HD digital picture signal, a digital VTR using a one-inch tape has been used for commercial purposes. However, a digital VTR has not been created which can record and reproduce both the current digital picture signal and the HD digital picture signal in a single machine.
When assumption is made that a home-use digital VTR is produced which is compatible with high definition television (HDTV) broadcasting expected to put into a full-fledged operation in the near future, it is naturally desired that the digital VTR can record and reproduce both the current digital picture signal and the HD digital picture signal with a common cassette tape and a common scanner (drum and heads). Further, as it is produced for home use, the VTR should be a less expensive system.
FIG. 14 shows a possible structure of recording/reproducing heads in the above-mentioned home-use digital VTR. Heads 20a and 20b of a double azimuth head are mounted 180.degree. opposite to heads 21a and 21b of another double azimuth head, so that the wrapping angle of the magnetic tape 3 is 180.degree. as in the ordinary VTR for home use. When the digital picture signal is used, the amount of information handled is greater, and therefore, even when the current digital picture signal is handled, in order to accommodate the narrower track for higher recording density and the increasing amount of information transmitted, the apparatus in general use in the future will be a type of two-channel simultaneous recording. With regard to this technique, refer to Yamashita et al. "AN EXPERIMENTAL STUDY ON BIT RATE REDUCTION AND HIGH DENSITY RECORDING FOR A HOME-USE DIGITAL VTR", IEEE Transactions on Consumer Electronics, Vol. 34, No. 3, August 1988, pp. 588-596, especially FIG. 6.
FIG. 15 shows the pattern (on magnetic face) of the signal tracks of the tape on which information is recorded with the heads structured as shown in FIG. 14. The + azimuth heads 20a and 21a and the - azimuth heads 20b and 21b are set which have a wide width with respect to the track pitch P in order to perform guard-bandless recording. In recording, if the heads 20a and 20b operating as one body start tracing from the point A, the heads 21a and 21b which come round after the drum rotates 180.degree. will start tracing from the point B. By tracing by the heads 21a and 21b, the previously recorded track is erased (overwritten by the head 21a) for a width corresponding to .DELTA.P in FIG. 15. There are differences in the mounted height between the heads 20a and 20b, and between the heads 21a and 21b (not at the same height). This height difference is set according to specifications of the tape speed and the head width so that uniform signal tracks with track pitches P of two channels are formed as illustrated. The L in FIG. 15 is the track length, which is substantially equal to the wrapping length of tape on the rotating drum 4 (diameter D).
FIG. 16 shows how the recorded tape in FIG. 5 is played back. In reproduction, by control (not shown) of a known capstan motor for driving the tape 3, the heads 20a, 20b and 21a, 21b trace along the centers of the recorded tracks. Further, during reproduction, information is outputted for two channels simultaneously as shown by (b) in FIG. 17 at the drum rotation periods T as shown by (a) in FIG. 17.
Let us consider how to make the conventional home-use digital VTR compatible with the HD digital picture signal by using the two-channel simultaneous recording/reproducing system in FIG. 14.
Even when it is assumed that the amount of information of the HD digital picture signal can be compressed to about twice as much as the current digital picture signal by employing the bit rate reduction technology, if attempts are made to provide compatibility with the HD digital picture signal by using the system in FIG. 14, it follows that twice as much information as with the current digital picture signal must be handled. As measures for this, there are two possible methods.
The first method is to control the switch-over of the number of drum revolutions to twice faster drum speed with respect to the current digital picture signal processing speed, thereby increasing the amount of information that can be recorded in the same period of time to twice as large as before.
The second method is to keep the same number of drum revolutions as with the current digital picture signal, double the number of heads, and control the switch-over of the number of the heads used.
FIG. 18 shows a first method by which to control switch-over of the number of drum revolutions, and indicates the state in which the number of drum revolutions is made twice faster (2.times.N) and accordingly, the tape feeding speed is made twice faster (2.times.V) when the HD digital picture signal is used than in the case where the current digital picture signal is handled as in FIG. 14. The other structural arrangement is the same as in FIG. 14. FIG. 19 shows how the signals are outputted during playback by the structural arrangement in FIG. 18. As the number of drum revolutions is increased twice as high as before, signals for four tracks same as in FIG. 17, that is, twice as much information can be handled in the drum rotation period of T/2, which is one-half of the drum rotation period T in playback of the current digital picture signal in FIG. 17. It ought to be noted that in recording the signal by the structural arrangement of FIG. 18, only the number of drum revolutions and the tape speed are made twice higher, but the other recording conditions remain unchanged as in the case of recording the current digital picture signal, in other words, the tracks on which signals are recorded are the same as in FIG. 15, and the track pitch, too, is the same as in the conventional arrangement in which the drum speed and the tape speed are half as high as in recording the current digital picture signal. In the structure in FIG. 18, the only changed recording condition is a decrease in the recording time resulting from the twice increased tape feeding speed. To be more specific, the recording time of the HD digital picture signal is reduced by half from the recording time of the current digital picture signal by the structure in FIG. 14.
FIG. 20 shows the second method mentioned above which increases the number of heads, and the only difference of this head structure from FIG. 14 is that addition of two pairs of double azimuth heads 22a, 22b and 23a, 23b disposed at 180.degree. opposite positions and at right angles with the heads 20a, 20b and 21a, 21b. When the current digital picture signal is processed, as in FIG. 14, only a pair of the heads comprising 20a, 20b and 21a, 21b is selected and used to perform the same operation shown in FIG. 17. When the HD digital picture signal is processed, another pair of heads 22a, 22b and 23a, 23b is used in addition to the pair of heads 20a, 20b and 21a, 21b, and thereby the number of heads used is doubled.
FIG. 21 shows how the signals are outputted during playback by the structual arrangement in FIG. 2.
However, the above methods respectively have problems. To be more specific, by the first method to control switch-over of the number of drum revolutions, the number of drum revolutions is increased to an ultra-high speed, that is, twice faster than several thousand rpm even when the current digital picture signal is processed, and what is more serious is that the frequency of the reproduced signal changes in the ratio of two to one. Here let us consider the frequency f of the signal reproduced by the heads. When the head scanning velocity is denoted by v and the recording wavelength by .lambda. (constant), the relation .nu.=f.times..lambda. is well known, and from this relation, if, for processing the HD digital picture signal, the number of drum revolutions is increased twice faster than when the current digital picture signal is processed, the head scanning speed v, too, becomes twice faster, so that the frequency of the reproduced signal becomes twice higher. Though the digital VTR is consequently compatible with different picture signals of two different systems, containing different amounts of information, it is necessary to provide a couple of head-reproduced signal waveform equalizer circuits and a couple of data strobe circuits.
Further, the second method has a fixed number of drum revolutions, and therefore, the frequency of the reproduced signal remains unchanged, but the second method requires eight heads, which requires the number of reproduction amplifiers to be increased. Another problem with the second method is complicated adjustment of the mounting heights of the heads for securing equal recorded track pitches. These problems are undesirable in respects of price and production.
The first and the second methods have a fatal defect attending on simultaneous recording of the two channels. Specifically, this is a problem of inability to accommodate the long-time play mode even though it is possible to achieve compatibility with the current and HD digital picture signals.
The long-time play mode is to cut in half the pitch of the recorded tracks by reducing the tape running speed by half, for example, while the normal number of drum revolutions is kept, and to thereby record twice larger amount of information as in the standard play mode. By the head structure for two-channel simultaneous recording or reproduction, when the number of drum revolutions is fixed and the tape speed is reduced by half, in the structure of FIG. 14, for example, as shown in FIG. 15, the tracing start point of the heads 21a, 21b is shifted from B to C, and furthermore, the tracing start point of the heads 20a, 20b which come round next is shifted successively in the leftward tape running direction of the diagram. Accordingly, as shown in FIG. 22, not only the track pitch of the + azimuth head is not narrowed to have the same pitch as in the standard play mode, but also only the track pitch of the - azimuth head is narrowed as it is erased by overwriting.
In order to solve this problem, a possible measure will be to mount a head dedicated to the long-time play mode separate from the standard play mode, but this simply makes the head structure more complicated, so that this measure has no realizability.